Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless communication devices, personal digital assistants (PDAs), laptop computers, desktop computers, digital cameras, digital recording devices, cellular or satellite radio telephones, and the like. Digital video devices can provide significant improvements over conventional analog video systems in creating, modifying, transmitting, storing, recording and playing full motion video sequences.
A number of different video encoding standards have been established for encoding digital video sequences. The Moving Picture Experts Group (MPEG), for example, has developed a number of standards including MPEG-1, MPEG-2 and MPEG-4. Other standards include ITU H.263, QuickTime™ technology developed by Apple Computer of Cupertino Calif., Video for Windows™ developed by Microsoft Corporation of Redmond, Wash., Indeo™ developed by Intel Corporation, RealVideo™ from RealNetworks, Inc. of Seattle, Wash., and Cinepak™ developed by SuperMac, Inc.
Many video encoding standards achieve increased transmission rates by encoding data in a compressed fashion. Compression can reduce the overall amount of data that needs to be transmitted for effective transmission of video frames. The MPEG standards, for example, utilize graphics and video compression techniques designed to facilitate video and image transmission over a narrower bandwidth than can be achieved without the compression.
The MPEG standards, for example, support video encoding techniques that utilize similarities between successive video frames, referred to as temporal or inter-frame correlation, to provide inter-frame compression. The inter-frame compression techniques exploit data redundancy across frames by converting pixel-based representations of video frames to motion representations. In addition, the video encoding techniques may utilize similarities within frames, referred to as spatial or intra-frame correlation, to further compress the video frames. Intra-frame compression is typically based upon texture encoding for compressing still images, such as discrete cosine transform (DCT) encoding.
To support compression, a digital video device typically includes an encoder for compressing digital video sequences, and a decoder for decompressing the digital video sequences. In many cases, the encoder and decoder form an integrated encoder/decoder (CODEC) that operates on blocks of pixels within frames that define the sequence of video images. In the MPEG-4 standard, for example, the encoder typically divides a video frame to be transmitted into macroblocks comprising 16 by 16 pixel arrays.
For each macroblock in the video frame, an encoder searches macroblocks of the immediately preceding video frame (or subsequent frame) to identify the most similar macroblock, and encodes the differences between the macroblocks for transmission, along with a motion vector that indicates which macroblock from the previous frame was used for encoding. A decoder receives the motion vector and encoded differences, and performs motion compensation to generate video sequences.
The video encoding process is computationally intensive, particularly when motion estimation techniques are used. For example, the process of comparing a video block to be encoded to video blocks of a previously transmitted frame requires large numbers of computations. Improved encoding techniques are highly desirable, particularly for use in wireless devices or other portable video devices where computational resources are more limited and power consumption is a concern. At the same time, improved compression is desirable to reduce the bandwidth required for effective transmission of video sequences. Improving one or more of these factors may facilitate or improve real-time encoding of video sequences, particularly in wireless and other limited-bandwidth settings.